Passive couplers for the polymer optical fiber may be used, for example, for transferring the data stream carried by optical signals from one POF to two or more POFs. Applications thereof are optical communication systems having a distribution function, measuring systems in which a permanent control of the optical power during operation must take place, or optical, bi-directionally operating communication systems, in which both incoming and outgoing signals are transmitted on a single POF (see in this respect W. Daum, J. Krauser, P. Zamzow, O. Ziemann: “POF Optische Polymerfasern für Datenkommunikation” , Springer Verlag 2001, pg. 150). In these bi-directional systems, the transmitter and the receiver of a network node are connected to the communication fiber via an optical splitter.
During the past 20 years or so, a large number of different technologies for the manufacture of optical splitters have been presented. Fiber-optical coupling elements, integrated optical coupling elements, as well as structures with injection-molded coupling elements are known. The so-called polished couplers are of special interest because of their structural simplicity and high optical quality (see the above-mentioned book of W. Daum et al.).
As implied by the name “polished coupler,” for the manufacture of a polished coupler, the end section of a POF may be planarly, high-quality ground in oblique direction to half of the diameter of the POF and subsequently may be joined to an end section of a second POF processed in the same way, so that the two semicircular flat end faces of the ground POFs resulting from the grinding processes may be combined to form a full circular end face, which after planar grinding of the flat surface exactly fits onto the flat end face of a third, exiting or incoming POF. The flat end faces abutting each other and the obliquely extending, polished surfaces usually are each adhered to one another by means of a transparent adhesive. In this method it is advantageous to create the surface of the obliquely extending polished portion, of each of the first mentioned POFs, which extends essentially in the axial direction of the third POF, in optical quality, i.e., with generally highly gloss finish. The light guided within the POF must not be scattered at this surface. The smallest roughness leads to stray light and thus to a quality loss of the optical splitter.
On the basis of the polished coupler, couplers with an excess loss of less than 1 dB can be manufactured. The excess loss describes the optical loss of the splitter exceeding the pure splitting loss. It is also not difficult to produce splitters having an almost arbitrary splitting ratio by asymmetrical high-quality grinding of the two or more POFs.
Up to now a series production of these splitters could not be established, since the polished splitters or couplers, as their name already implies, typically must be manufactured by wet grinding on a water-rinsed polishing machine to obtain an optical high-quality surface. In an example of such a process, the POFs are first of all clamped into a polishing holder, this holder is then manually set onto the polishing machine, and subsequently the POF is first of all processed by coarse polishing paper, later with finer and finer polishing paper to its final dimension. Each change of the polishing paper must be made manually. The polishing paper has a limited service life and is also relatively expensive so that it contributes to a significant part of the overall costs in mass production.
Tests to substitute the above-mentioned wet polishing process with different machining methods usually fail because the required precision cannot be attained and the surface quality is insufficient as long as tools of tool steel or of hard metal are used.
However, it is known from industrial engineering of optical elements such as mirrors of non-iron materials or plastic lenses, that when using diamond cutters and diamond cutting chisels, shining optical surfaces can be created that do not have to be polished in addition to the cutting. It is also known that by the use of a very fast rotating diamond cutter, the end face of a POF can be worked such that a shining surface is created. It is also known from U.S. Pat. No. 6,473,555 to vary the cross-section of a POF by heating same and by pressing it into a mold of the desired cross section. However, this method is not suitable for removing part of the cross section of a POF which in the remainder shall remain unchanged.
To address such problems and others, it is an object of the invention to provide a method and a device whereby polymer optical fibers can be processed economically in large numbers and at high quality in a manner so that they can subsequently be mounted to form low-loss couplers or splitters.